The present invention relates to fringe deflectometry apparatus, comprising means for illuminating an optical component to be measured with radiation of a known wavefront; means for deflectometric measurement of said radiation after reflection or transmission thereof by said optical component to be measured, and means for materializing a reference beam.
The invention also relates to a method for fringe deflectometry, comprising steps consisting in illuminating an optical component to be measured with radiation having a known wavefront, and analyzing, in deflectometric measurement means, the fringes generated by the radiation reflected or transmitted by said component.
The invention also covers the application of this method to the measurement of ophthalmic lenses, lenses having a refractive index gradient and ophthalmic lens molds.
The methods of the invention apply in fringe deflectometry apparatus in which there is no phase reference, such as for example differential interferometry, moire pattern deflectometry or deflectometry by the Ronchi method.
A deflectometry apparatus using reflection or transmission is disclosed in FR-A-2,710,162 and EP-A-0,644,411 in the name of the applicant. This apparatus makes it possible to measure, using reflection or transmission, the geometrical structure of an optical component.
The principle of such a measurement apparatus is that of illuminating the optical component to be measured, using radiation having a known wave surface, and, in the simplest case, illuminating it with a spherical wave, and then measuring the shape or form of the wave surface after reflection or transmission at the optical component to be measured. Wave surface measurement after reflection or transmission makes it possible to work backwards to arrive at the geometrical characteristics of the component to be measured.
The invention applies also to the apparatus described in FR-A-2,747,912 and FR-A-2,647,913.
Reference is explicitly made to the above documents for more details regarding the measurement principle and a possible device for carrying out such measurement.
The invention applies to fringe deflectometry apparatus, using transmission as well as reflection. Generally speaking, it makes it possible to measure the inclination or tilt of a surface of a wave with respect to the optical axis of the apparatus. Such inclination results, in a fringe deflectometry method, in a phase term comprised between -.pi. and .pi., which is determined by phase detection, and by a multiple of 2.pi. phase term, which is not given by the phase detection method. The invention makes it possible to overcome this uncertainty.
Where the setup described in documents FR-A-2,710,162 and EP-A-0,644,411 is employed using reflection, it is not necessary to measure the inclination of the wave surface after reflection; autocollimation at one point on the surface makes it possible to provide a phase reference. The invention can allow this operation to be avoided.
When the setup in FR-A-2,710,162 and EP-A-0,644,411 is used in transmission, a new technical problem arises.
This problem is that of determining the prism that exists between the two faces of the component. The devices of the above documents can indeed operate in transmission, for components having an optical axis, always provided the position of this optical axis is known--in this case, the prism on the optical axis is known and is equal to zero, thereby providing a phase reference. If the optical axis is not known, or if the component has no optical axis, one could imagine, if needs be, measurement of the prism by introducing prism gauges following the component to be measured; this method would suffer from a lack of accuracy, would require time to implement, and would additionally involve aligning the first face of the component.
To sum up, known apparatuses, when used in transmission, do not make it possible to readily achieve measurement of the prism existing between the input and output faces of the component. This new problem occurs for any component, notably for measurement of a constant index dioptric component, or for a parallel-sided graded index plate. The solution to this problem amounts to supplying, in deflectometric apparatus using transmission, a phase reference for the deflectometric analysis means.
The invention sets out to resolve these technical problems.